Sheet material especially useful for circuit boards

ABSTRACT

A sheet comprising thermoplastic polymer (TP) and short high tensile modulus fibers, in which the concentration of TP in the middle of the sheet is higher than at the surface of the sheet, useful for making prepregs with a thermoset resin.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/315,885, filed Aug. 30, 2001.

FIELD OF INVENTION

[0002] The field of invention relates to sheets comprising athermoplastic polymer having low moisture absorption, and high tensilemodulus fibers, prepregs made therefrom, substrates for circuit boardsand other devices made therefrom, and methods for making the foregoing.

BACKGROUND

[0003] Circuit boards are important items of commerce, being used invirtually every electronic device. The “board” or supporting member of acircuit board and other electronic devices (such as the interposer in aflip-chip package) is an important component of such devices, andproperties of the materials used to make such boards are important tothe functioning of the electronic or electrical circuit. As electroniccomponents have become more sophisticated, the demands placed upon thematerials used for boards have increased. For example, for manyapplications it is preferred that the board have a coefficient ofexpansion which matches those of the chips mounted on the board, and/orthat the board have a low dielectric constant and a low dissipationfactor, especially when high frequency devices are mounted on the board.These three factors are often adversely affected by the absorption ofmoisture by the board materials, which changes the dimensions of theboard and/or changes the dielectric constant and dissipation factor ofthe board itself, and/or causes warpage.

[0004] The simplest boards for relatively nondemanding applications aretypically made from a thermoset resin such as an epoxy filled with afibrous reinforcement such as glass fiber. The glass fiber, often in theform of a woven fabric, is saturated with liquid epoxy resin to form a“prepreg”, which is cured in the form of a board. As the demands onboards increase, the glass may be replaced by a higher modulus infusiblefiber such as an aramid. However, fibers such as aramid fibers, andepoxy resins, absorb significant amounts of moisture, and so aresometimes unsuitable for use together in highly demanding circuit boarduses. Thus, there is a need for improved circuit board materials havingreduced moisture absorbance properties.

[0005] Japanese Patent Application 2000-334871 describes the preparationof a sheet from which a prepreg may be formed by “laminating” athree-layer structure in which the middle layer may be a nonwoven sheetcontaining synthetic organic fiber and the two outer layers may containaramids or other infusible fibers. While this reference discloses thatthe two outer layers may contain synthetic organic fiber in an amountless than that contained in the inner layer, no mention is made that thelaminated sheet contain a nonuniform distribution of synthetic organicfiber through the thickness of the laminated sheet.

[0006] Japanese Patent Application 11-117184 describes the preparationof a sheet from which a prepreg may be formed by forming a nonwovensheet from aramid and LCP (liquid crystal polymers) fibers, heating thesheet under pressure to make the LCP flow, and then adding a thermosetresin to form a prepreg. Nothing is said about variation of the LCPconcentration across the thickness of the sheet.

[0007] Japanese Patent Application 9-21089 describes the preparation ofan LCP nonwoven sheet (paper) which is reported to have low waterabsorption. Other fibers can also be present in the sheet. The product,after being heated under pressure to partially consolidate the sheet, isapparently still a paper-like material.

[0008] Japanese Patent Application 11-229290 describes the preparationof a paper made from LCP and aramid fibers which can be impregnated withan epoxy resin which is then cured. The resulting board may be used as acircuit board. No melting or flow under heat and/or pressure of the LCPis described.

SUMMARY OF INVENTION

[0009] Our invention includes:

[0010] A sheet comprising (a) one or more nonwoven sheets comprisingshort high tensile modulus fibers, and (b) thermoplastic polymer havinglow moisture absorption; wherein at least a portion of saidthermoplastic polymer is bound to at least some of said high tensilemodulus fibers; and through a cross section of the thickness of saidsheet, a concentration of said thermoplastic polymer, relative to atotal concentration of said short high tensile modulus fibers is greaterat a center of thickness of the sheet than at an outer surface of thesheet.

[0011] Structures containing one or more of such sheets together withuncured or cured thermoset resins and/or metal sheets are alsodescribed, as are circuit boards comprising these structures.

[0012] Also described are processes for making such sheets andstructures. The sheets are made by subjecting the thermoplastic polymerand one or more nonwoven sheets comprising one or more of short lengthsof high tensile modulus fibers to controlled heat and pressure, such as:

[0013] A process for the production of a first sheet material,comprising, heating and applying pressure for a sufficient amount oftime to a multilayer second sheet structure, comprising, at least twolayers of a nonwoven fabric of short high tensile modulus fibers, and atleast one layer containing a thermoplastic with low moisture absorption,provided that the two outer layers of said second sheet structure aresaid nonwoven fabric, to produce said first sheet material in which:

[0014] at least a major portion of said thermoplastic polymer is boundto at least some of said of high tensile modulus fibers; and

[0015] through a cross section of said first sheet, from the center ofthickness of said first sheet to both of the surfaces of said sheet, aconcentration of said thermoplastic polymer relative to a concentrationof high tensile modulus fibers, decreases.

BRIEF DESCRIPTION OF THE FIGURES

[0016]FIG. 1 shows a typical three layer structure made from two outershort high tensile modulus fiber (HTMF) nonwoven layers and athermoplastic (TP) film inner layer before being subjected to heat andpressure.

[0017]FIG. 2 shows the three layer structure of FIG. 1 after beingsubjected to heat and pressure to cause the TP present to partially flow“in between” some of the HTMF present.

[0018]FIG. 3 shows a three-layer structure made from two outer HTMFnonwoven layers and an inner layer which contains TP powder before beingsubjected to heat and pressure.

[0019]FIG. 4 shows the three-layer structure of FIG. 3 after beingsubjected to heat and pressure to cause the TP present to partially flow“in between” some of the HTMF present.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0020] Herein certain terms are used. Some of these are defined below.

[0021] By a “thermoplastic polymer having low moisture absorption” (TP)is meant a thermoplastic plastic polymer which absorbs less than 1.0weight percent moisture (based on the weight of the thermoplasticpolymer) when measured on a sheet of pure thermoplastic polymer by themethod described below. Preferably the moisture absorption of thethermoplastic polymer is about 0.5 weight percent or less, morepreferably about 0.25 weight percent or less, and especially preferablyabout 0.10 weight percent or less.

[0022] By “high tensile modulus fibers” (HTMF) are meant these productforms having a tensile modulus of about 10 GPa or more, preferably about50 GPa or more, more preferably about 70 GPa or more, when measured inaccordance with ASTM D885-85 method, using a 1.1 twist multiplier. HTMFherein include high tensile modulus fibers, fibrils and fibrids, unlessit is specifically indicated not all three are included.

[0023] By a TP being “bound” to a fiber-like material (as in saidthermotropic liquid crystalline polymer is bound to at some of said ofHTMF) is meant that the TP is “sticking to”, contacting over asubstantial part of an individual fiber surface, or encapsulatingindividual fibers. Any TP which is part of a mass (single “piece” orconnected network of TP) of TP which is bound to a fiber is consideredbound to fiber. Preferably the TP is bound to the fiber by melting orplastic deformation (flow at a temperature below the actual meltingpoint of the TP) of the TP that causes the LCP to contact the HTMFmaterial. For example heating and optionally applying pressure to amixture of an HTMF and a TP, will cause the TP to flow around the HTMFand result in at least part of the TP being bound to the HTMF.

[0024] By “nonwoven HTMF or aramid sheet” or “nonwoven HTMF or aramidfabric” is meant a nonwoven sheet (or fabric) that contains at least 70percent by volume of HTMF (or specifically aramid) fiber.

[0025] By “nonwoven sheet” herein is meant a nonwoven “fabric” formed byany number of different methods, for example wet lay of short fibers(often called a paper), dry lay, flash spun, melt spun, mechanicallyneedled felt, spunlaced. A preferred form of nonwoven sheet is a paperas described in U.S. Pat. Nos. 4,886,578 and 3,756,908, each of which ishereby incorporated by reference in its entirety. This process alsoincludes the optional use of a binder, wherein such binders include, butare not limited to, aramid fibrids as other binders known within theindustry may also be used in the process. Dry-laid methods ofmanufacturing which are well known within the art are described by U.S.Pat. No. 3,620,903, which is hereby incorporated by reference in itsentirety.

[0026] By “fiber” is meant an object having a length and a maximumcross-sectional dimension, the maximum cross sectional dimensiontypically being in the range of about 0.3 μm to about 100 μm and anaspect ratio (length/width) of ≧50.

[0027] By “aramid fiber” herein is meant aromatic polyamide fiber,wherein at least 85% of the amide (—CONH—) linkages are attacheddirectly to two aromatic rings. Optionally, additives can be used withthe aramid and dispersed throughout the polyfiber structure, and it hasbeen found that up to as much as about 10 percent by weight of otherpolymeric material can be blended with the aramid. It has also beenfound that copolymers can be used having as much as about 10 percent ofother diamines substituted for the diamine of the aramid or as much asabout 10 percent of other diacid chlorides substituted for the diacidchloride of the aramid.

[0028] By “fibrils” herein is meant a fiber-like material having adiameter of about 0.1 μm to about 25 μm, and an aspect ratio of 3 toabout 100.

[0029] By “fibrids” herein is meant very small, nongranular, fibrous orfilm-like particles with at least one of their three dimensions to be ofminor magnitude relative to the largest dimension. These particles areprepared by precipitation of a solution of polymeric material using anon-solvent under high shear.

[0030] By “LCP” is meant a liquid crystalline polymer which isanisotropic when tested by the TOT test as described in U.S. Pat. No.4,118,372, which is hereby incorporated by reference in its entirety. Bythermotropic is meant the LCP may be melted and is anisotropic in themelt, as described in the TOT test.

[0031] The term “aramid fibrids”, as used herein, means non-granularfilm-like particles of aromatic polyamide having a melting point ordecomposition point above 320° C. The aramid fibrids typically have anaverage length in the range of about 0.2 mm to about 1 mm with an aspectratio of about 5 to about 10. The thickness dimension is on the order ofa fraction of a micrometer, for example about 0.1 μm to about 1.0 μm. Inaddition to aromatic polyamide, aramid fibrids may optionally compriseone or more of dyes, pigments or some other additives such as thosedescribed in U.S. Pat. Nos. 5,965,072 and 5,998,309, each of which ishereby incorporated by reference in its entirety.

[0032] By “short fibers” or “short lengths” of fibers herein is meantfibers with an aspect ratio preferably of less than about 2000, morepreferably about 200-1000, and even more preferably about 250-600.

[0033] By “powder” herein is meant a material having an aspect ratio ofless than 3. These particles typically have a maximum dimension of about5 μm to about 1000 μm.

[0034] By “a” or “an” herein, such as a TP or HTMF is meant one or more.

[0035] By “comprising” herein is meant the named items (materials), andany other additional materials or compositions may be present.

[0036] Preferred methods for making preferred first sheets of ourinvention are now described.

[0037] One preferred method comprises forming the first sheet from athree layer structure comprising two outer layers and an inner layertherebetween.

[0038] The inner layer preferably contains a relatively large amount ofTP. The TP may be present in the inner layer as a film, paper, shortfiber, fiber, fibrid, fibril, or powder, or any combination of these.Most preferably, the TP is present in the inner layer as a film, paper,or powder. Even more preferably, the inner layer comprises TP film or TPpowder. For LCPs, because of the tendency of solid LCPs to fibrillatewhen worked mechanically, combinations of the above forms when the LCPis in particulate form may be employed. LCPs which are particulates anddo not match any of the above particulate definitions may also be used.

[0039] The inner layer may also contain some HTMF, for example, it maybe an HTMF paper which contains a TP powder which was co-laid with thepaper or worked into the paper after the HTMF paper was formed.Alternatively, the inner layer may be a paper containing both HTMF andTP, especially LCP fibers, with a relatively large amount of TP present,or the inner layer may be a TP, especially an LCP paper containing anLCP powder.

[0040] The two outer layers are preferably nonwoven HTMF sheets,preferably HTMF papers. They may contain some relatively [compared tothe inner layer(s)] small amount of TP, for example in the form of shortfibers, fibrils, and/or powder. The inner layer may contain some HTMF,and the outer layers may contain some TP, but the inner layer must behigher in TP concentration than the outer layers.

[0041] All of these layers may contain other items, such asantioxidants, pigments, inorganic fillers, and colorants. Preferably,none of these additional items should significantly deleteriously affectthe overall performance in the final circuit board.

[0042] Other preferred processes for making the first sheet includeincorporating the inner (TP, especially LCP, rich) layer in a sandwichstructure as a film, a paper, a woven fabric, a resin-rich sheetutilizing other fiber for structural integrity or a non-woven batting orsheet. In addition, the TP could be added directly as a powder or otherparticulate form through weigh feeding or other controlled meteringequipment. The sandwich structure would then be processed with heat andpressure by a press, autoclave, calender rolls or belt press. Acontinuous process can be envisioned where the resin is introduced as amelt or powder between the two outer (fiber-rich) layers into theentrance nip of a calender rolls or a belt press where heat and pressureis applied. A vacuum applied to the outer side of each fiber-rich outerlayer may be used to assist the flow of molten resin into the sheetbefore pressure is applied, thus setting up the concentration gradient.

[0043] In another variation on this method one inner layer (TP rich) andone outer layer (HMTF rich), as described immediately above, are firstbonded together, and then two of these bilayer sheets are in turn bondedtogether with the two original “inner layers” of these bilayer sheetsfacing each other. These two bilayer sheets are then bonded together. Inessence the two “inner layers” of the two bilayer sheets which have beenbonded now become a single inner layer, and the original outer layersare in fact the outer layers of the final first sheet produced.

[0044] Typically an inner layer will preferably comprise about 20 toabout 80 percent by weight, more preferably about 30 to about 50 percentby weight, of TP, based on the total weight of HTMF and TP in the threelayer structure. An HTMF paper preferably weighs about 15 to about 200g/m². The thickness of the inner layer will be dependent upon its form,for example a film will be thinner than an equivalent (in g/m²) paper orpowder. The structure may have more than three layers, so long as thefinal first sheet has the required attributes.

[0045] If another resin, such as a cured or uncured thermoset resin ispresent, only that part of the sheet which contains TP and/or HTMF (andnot only “pure” thermoset) is considered as part of the thickness of thesheet for the purposes of determining relative TP and HTMFconcentrations. Where only one or more other resins are present (no HTMFand/or TP), the ratio or relative concentrations of TP and/or HTMF arein a sense meaningless.

[0046] A cross section of a preferred three layer structure is shown inFIG. 1. It is noted that each of the Figures is not to scale, and theorientation of the aramid fibers shown therein does not necessarilyrepresent a true orientation, but is for illustrative purposes only.Referring to FIG. 1 (shown greatly enlarged through the thickness of thestructure), both outer layers 1 are HTMF paper layers, consisting ofshort HTMF fibers. Inner layer 2, in the middle, is a TP film. In orderto form the first sheet, the three layer structure, for example as shownin FIG. 1, is subjected to heat and pressure for a sufficient amount oftime to cause at least some of the TP to flow into part of the each ofthe outer layers 1. This then forms a first sheet as shown in FIG. 2.Here the HTMF paper layers 3 have been partially penetrated by the TP 4.Note that in this illustration only some of the TP has penetrated intoeach of the HTMF layers, therefore forming a TP concentration gradientin this first sheet. The concentration at the center is 100% TP, whileat the outer surfaces of both of outer layers 3 the concentration of TPis zero. If the amount of TP present is small enough there simply maynot be enough present to completely fill the voids in the HTMF layers.Note however that if a high enough temperature is applied for a longenough time, the TP may flow enough to form a sheet which is homogeneouswith respect to the relative concentrations of TP and HTMF through thethickness of the sheet.

[0047]FIG. 3 shows another preferred three layer structure, with twoouter layers of HTMF paper 5, and an inner layer 6 comprising TP powder.FIG. 4 shows the first sheet formed from the three layer structure ofFIG. 3 by the application of heat and pressure, in which the HTMF paperlayers 7 have been partially penetrated by the TP 8, now mostlyconsolidated.

[0048] Preferably, at least a portion of the TP is bound to at leastsome of the HTMF in the resulting first sheet; more preferably, a majorportion (i.e., more than half) of the TP in the first sheet is bound toat least some of the HTMF; and even more preferably, essentially all ofthe TP in the first sheet is bound to at least some of the HTMF.

[0049] For example, in FIG. 2, essentially all of the TP present isbound to HTMF, since the TP started as a film which flowed into part ofthe HTMF paper layers present. In this representation it is assumed TPis interconnected as sort of network, and some of the TP is touching orencapsulating the HTMF (is bound to the HTMF), and so all of the TP isbound to the HTMF. On the other hand in FIG. 4, some of the TP powderparticles are shown as not being connected (presumably did not flow) toTP polymer which is bound to HTMF, and so these unconnected particlesare not considered bound to the HTMF. The TP which did flow into theHTMF are bound to those fibers.

[0050] In the first sheet, through a cross-section of the first sheet, aconcentration of TP, relative to the total concentration of the HTMF, isgreater at the center of thickness of the first sheet than at an outersurface of the first sheet. More preferably, the concentration of TP atthe center of thickness of the first sheet is greater than at both outersurfaces of the first sheet. Even more preferably, when measured fromthe center of thickness to either outer surface, the concentration of TPexhibits a gradient. The gradient is preferably a generally decreasingfunction from the center to the outer surface. For example, the gradientmay exhibit step changes of decreasing order or may be continuously(smooth) decreasing or may exhibit a series of increases and decreasesalong an extrapolated decreasing gradient (smooth or step change).

[0051] For example, in FIGS. 2 and 4, the concentration gradient of theTP does not have to be a smooth gradient. Rather the gradient asmentioned for the first sheet is over the entire thickness from thecenter of the sheet to the surface (except that portion which containsonly thermoset resin). There may be an abrupt change in theconcentrations, as shown in FIG. 2.

[0052] In a preferred first sheet, the concentration of TP in the center10% of the first sheet (5% either side of the center) is preferablyabout 20% or more, more preferably about 30% percent or more, andespecially preferably about 40% or more. In another preferred sheet theconcentration of HTMF at 10% of the thickness of the sheet measured froma surface of the sheet is 100% HTMF (no TP), or preferably about 50% ormore, more preferably about 75% or more, and especially preferably about95% or more. All percentages in this paragraph are by volume, based onthe total amount of HTMF and TP present in the “thickness layer”specified.

[0053] Conditions for forming the first sheet are a combination oftemperature (heating), pressure and the amount of time heating andpressure are applied. Generally, the higher the temperature applied, theless the pressure needed and/or the less time needed. The higher thepressure, the lower the temperature needed and/or less time is needed.The longer the time used, the lower the temperature and/or the lower thepressure which may be needed. However, in most cases it may be necessaryto heat the TP to at least near its melting point. If too high atemperature, or too high a pressure, or too long a time, or anycombination of these is used, the TP may flow enough to form anessentially uniform (through the thickness of the sheet) compositionwith the HTMF. In this case, the temperature and/or pressure should belowered and/or the time shortened. If the TP exhibits too little flow,i.e., basically remains a separate layer in the center, then thetemperature and/or pressure should be raised, and or the time increased.It is believed that the most important variable is temperature,particularly when approaching the melting point of the TP.

[0054] A variety of methods can be used to apply both highertemperatures and pressures. A simple apparatus is a vacuum bag to whichheat and pressure may be applied. A press or autoclave may be used.Preferred methods are hot roll and hot belt calendering. Temperatures,pressures, and time of treatment (contact) with the hot roll(s) orbelt(s) can be controlled fairly well, as can the final thickness of thefirst sheet. Calendering is a well known art, see for instance U.S. Pat.No. 3,756,908, which is hereby incorporated by reference in itsentirety.

[0055] Any TP which has a low moisture absorption, such asperfluorothermoplastics [for example, polytetrafluoroethylene;copolymers of tetrafluoroethylene with hexafluoropropylene,perfluoro(vinyl ethers) such as perfluoro(methyl vinyl ether)], orethylene; poly(ether-ether-ketones); poly(ether-ketone-ketones); andpoly(ether-ketones); polyesters such as poly(ethylene terephthalate),poly(ethylene 2,6-napthalate), poly(bisphenol-A isophthalate),poly(bisphenol-A isophthalate/terephthalate); polycarbonates; poly4-methylpentene; syndiotactic polystyrene; poly(aryl sulfides);poly(ether-imides); poly(aryl ethers); and LCPs are useful. PreferredTPs are perfluoropolymers, particularly those mentioned above, LCPs, andpolyesters, and LCPs are especially preferred. Among the preferredproperties for the TPs are high melting point, low dielectric constantand low dielectric loss coefficient.

[0056] LCPs useful herein include those described in U.S. Pat. Nos.3,991,013, 3,991,014, 4,011,199, 4,048,148, 4,075,262, 4,083,829,4,118,372, 4,122,070, 4,130,545, 4,153,779, 4,159,365, 4,161,470,4,169,933, 4,184,996, 4,189,549, 4,219,461, 4,232,143, 4,232,144,4,245,082, 4,256,624, 4,269,965, 4,272,625, 4,370,466, 4,383,105,4,447,592, 4,522,974, 4,617,369, 4,664,972, 4,684,712, 4,727,129,4,727,131, 4,728,714, 4,749,769, 4,762,907, 4,778,927, 4,816,555,4,849,499, 4,851,496, 4,851,497, 4,857,626, 4,864,013, 4,868,278,4,882,410, 4,923,947, 4,999,416, 5,015,721, 5,015,722, 5,025,082,5,086,158, 5,102,935, 5,110,896, 5,143,956, and 5,710,237, each of whichis hereby incorporated by reference in its entirety, and European PatentApplication 356,226. Preferably the TP (especially an LCP) has a meltingpoint of about 180° C. or more, very preferably 250° C. or more, morepreferably about 300° C. or more, and especially preferably about 325°C. or more. Melting points are determined by ASTM D3418-82, at a heatingrate of 20° C./min. The peak of the melting endotherm is taken as themelting point. These higher melting TPs will allow the circuit board toundergo high temperature processing with less possibility of warping,for example in reflow soldering. Low warpage is an important attributeof the boards used in circuit boards. Another preferred form of LCP isan aromatic polyester or aromatic poly(ester-amide), especiallypreferred are aromatic polyesters. By an “aromatic” polymer is meantthat all of the atoms in the main chain are part of an aromatic ring, orfunctional groups connecting those rings such as ester, amide, or ether(the latter of which may have been part of a monomer used). The aromaticrings may be substituted with other groups such as alkyl groups. Someparticularly preferred aromatic polyester LCPs are those found in U.S.Pat. Nos. 5,110,896 and 5,710,237 listed above. More than one LCPcomposition may be present in the first sheet, but one is preferred.

[0057] The TP may be present in the form of fibers, short fibers,fibrids or fibrils, and any one or more of these may be formed into anonwoven sheet, with or without other fibers (e.g., HTMF) also beingpresent in the sheet. “Fiber-shaped” LCPs may be formed simply by wetpulping of pieces of LCPs such as pellets. For example the pellets aremixed with moisture, and if desired one or more surfactants, and themixture subjected to relatively high shear mixing. If the shear appliedis high enough the pellets will be broken up into LCP fiber-likeparticles. Other forms of LCPS, particularly particulate forms, may beused.

[0058] Useful HTMFs include organic fibers such as aramids,poly(phenylenebenzobisoxazole), poly(phenylenbenzobisimidazole),poly(phenylenebenzobisthiazole), poly(phenylene sulfide), LCPS, andpolyimide, and also inorganic fibers such as glass fibers, siliconcarbide, boron nitride, alumina and other whiskers, and Wollastonite.When calculating the concentration of such fibers, the total of thesetypes of fibers present will be used, for example the total of aramidand poly(phenylenebenzobisoxazole) fiber present. Among the preferredproperties are high modulus, high melting point and/or glass transitiontemperature and low moisture absorption.

[0059] Aramids, poly(phenylenebenzobisoxazole),poly(phenylenbenzobisimidazole), poly(phenylenebenzobisthiazole), arepreferred HTMFs, and aramids are more preferred. Useful aramids includepoly(p-phenylene terephthalamide), poly(m-phenylene isophthalamide, andpoly(p-phenylene/oxydianiline terephthalamide) copolymers. Preferredaramids are poly(p-phenylene terephthalamide), poly(m-phenyleneisophthalamide), and poly(p-phenylene terephthalamide) is especiallypreferred. A description of the formation of aramid (short) fibers,fibrids and fibrils of various types is found in U.S. Pat. Nos.5,202,184, 4,698,267, 4,729,921, 3,767,756 and 3,869,430, each of whichis hereby incorporated by reference in its entirety. Description of theformation of nonwoven aramid sheets, especially papers, is found in U.S.Pat. Nos. 5,223,094 and 5,314,742, each of which is hereby incorporatedby reference in its entirety. More than one HTMF, including more thanone aramid may be present in the first sheet.

[0060] Once the first sheet is formed, it may be impregnated with athermoset resin. Before impregnation the first sheet may be treated toimprove the adhesion of the thermoset to the LCP and/or aramid; forexample, the first sheet may be surface treated by corona discharge by aplasma treatment. Since the surface of the first sheet will usually beporous to some extent (due to incomplete or no coating of the HTMFs bythe TP) the (usually) uncured liquid thermoset resin will penetrate thesurface of the first sheet and form an outer layer of uncured thermosetresin on the first sheet. This is termed herein the “prepreg”. Thethermoset resin may be cured on a single layer of the prepreg, or morethan one layer may be stacked together and cured together to form athicker board. Herein all such cured sheets are called second sheets. Ametal such as copper may be placed on one or both surfaces before curingthe thermoset resin. This is termed herein the “laminate”. Preferredthermoset resins are epoxy resins, polyimides, cyanurate esters andbismaleimide-triazine resins; epoxy resins and bismaleimide-triazineresins are especially preferred.

[0061] Circuit boards (including printed wiring boards and printedcircuit boards) produced from second sheets and/or laminates usuallyhave low moisture absorption, and/or good high temperature resistance,and/or relatively low coefficients of thermal expansion, and/or lowdielectric constant, and/or low warpage, and/or low dielectric losscoefficients, an excellent combination of properties for a circuitboard. Once the substrate boards are formed they may be processed bynormal methods to make circuit boards. The substrate boards could alsobe processed by normal methods to make useful composite parts, such asradar domes.

[0062] Second sheets containing one or more layers of first sheets mayalso be used in or as chip package substrates, chip carriers and chippackage interposers.

[0063] Second sheets may also be combined in laminates with other typesof sheets, for example glass fiber prepreg or RCF, for other uses. Forexample this “cored” structure may be used in printed wiring boards andfor chip packaging.

EXAMPLES

[0064] The following Examples 1-12 illustrate preferred embodiments ofour invention. Our invention is not limited to these Examples 1-12.

[0065] Method for determination of moisture absorption at 850C and 85%humidity: Five specimens (5×5 cm) of the same sample are dried toconstant weight at 105° C. in air, and are placed into a humiditychamber set for 85° C. and 85% humidity. After that, weight gain of thespecimens is measured each day. When an average weight gain for threeconsecutive days is less than 1% of the total weight gain, specimens aredeemed to be saturated and average moisture absorption is calculated bydividing the total weight gain by the original weight of the sample andmultiplying the result by 100.

[0066] In the Examples, except as noted, all of the LCP used had thecomposition as that of Example 4 of U.S. Pat. No. 5,110,896 i.e. derivedfrom hydroquinone/4,4′-biphenol/terephthalicacid/2,6-naphthalenedicarboxylic acid/4-hydroxybenzoic acid in molarratio 50/50/70/30/320.

[0067] Also in the Examples herein the poly(m-phenylene isophthalamide)fibrids were made as described in U.S. Pat. No. 3,756,908, which ishereby incorporated by reference in its entirety. The poly(p-phenyleneterephthalamide) had a linear density of about 0.16 tex and a length ofabout 0.67 cm (sold by E.I. du Pont de Nemours and Company undertrademark KEVLAR® 49).

[0068] Poly(ethylene terephthalate) (PET) fiber used: 2.1 dpf, 6 mmlong, sold by E. I. DuPont de Nemours & Co., Inc. Wilmington, Del.,U.S.A., as merge 106A75

[0069] Glass fiber used: E-type glass fiber 6.5 μm diameter and 6.4 mmlong produced by Johns Manville Co., Denver, Colo. 80217, USA, sold astype M189.

[0070] Poly(phenylene oxide) (PPE) resin used was type 63D from theGeneral Electric Co., Pittsfield, Mass., U.S.A.

[0071] The epoxy resin system used was grade L-1070, supplied by FortinIndustries.

Example 1

[0072] A multilayer structure of:

[0073] (1) As-formed paper (68 g/m² basis weight) made from 87% byweight poly(p-phenylene terephthalamide) floc (2.25 denier per filament,6.7 mm cut length) and 13% by weight poly(m-phenylene isophthalamide)fibrids.

[0074] (2) A 56 μm thick film of LCP. The film had a basis weight of 78g/m².

[0075] (3) Same as layer 2

[0076] (4) Same as layer 1.

[0077] was prepared [layers (1) and (4) formed the outer surfaces] andpassed through a 76.2 cm diameter roll calender at 305 linear cm/minutewith the rolls at 350° C. and at a pressure of 263 kN/m of width. Theresulting sheet was densified and bonded together. Photomicrographsshowed that the LCP layers had flowed into the aramid paper layers,penetrating almost to the outer surfaces but leaving a small quantity ofthe LCP resin in the middle of the structure. There was no evidence ofdisplacement of the p-aramid fibers by the penetrating resin. Theproduct had a basis weight of 254 g/m², was 292 μm thick, and anapparent density of 0.87 g/mL. It had a coefficient of thermal expansionof 1.80 ppm/° C. in the machine (calender) direction, and −1.83 ppm/° C.in the transverse direction.

[0078] When the same experiment was run only at a roll speed of 183linear cm/min (in other words a longer contact time with heat andpressure) the LCP had penetrated to within one aramid fiber thickness ofthe surface, leaving no essentially pure LCP layer of LCP in the sheet.This sheet material had an apparent density of 0.68 g/mL. Thisillustrates one method of controlling whether an LCP concentrationgradient will be obtained.

Example 2

[0079] LCP blown film (with nominal thickness of 30 μm and average basisweight of 41 g/m²) was laminated between two layers of formed paper(same as Layer 1 in Example 1 with basis weight of 31 g/m²) underfollowing conditions in a vacuum press:

[0080] (a) held in vacuum (no external pressure or temperature).

[0081] (b) Heated to 320° C. (5° C./min) from ambient condition under apressure of 6.9 MPa.

[0082] (c) Held for 1 h at 320° C. and 6.9 MPa.

[0083] (d) Cooled to rapidly room temperature (water quench) underpressure

[0084] Laminates were produced with average dimension of 25 cm×20 cm×185μm and average weight of 4.7 g (basis weight 100 g/m²). The laminateswere approximately 60% paper aramid and 40% LCP

[0085] The laminates from made above were prepregged with a commercialmulti-functional epoxy using standard techniques known in the industry.The resulting three component composite (paper+LCP film+epoxy) was 235μm thick with an average epoxy content of 46% by weight of the totallaminate. The final laminate had approximately 32% aramid, 22% LCP filmand 46% epoxy, by weight.

[0086] Six of the prepregs were further laminated between two sheetscopper (17 μm thick) under following conditions in a vacuum press:

[0087] (a) held in vacuum (no external pressure or temperature)

[0088] (b) Heated to 200° C. (5° C./min) from ambient temperaturecondition under a pressure of 6.9 MPa.

[0089] (c) Held for 1 h at 200° C. and 6.9 MPa.

[0090] (d) Cooled rapidly to room temperature (water quench) underpressure.

[0091] The laminates had an average thickness of 850 μm (6 prepregs+2 Cusheets). After etching away the copper foils, the properties of thepolymeric portion of the laminate were measured. Average coefficient ofthermal expansion in plane was about 12 ppm/° C. and moisture absorptionat 85° C. and 85% humidity was 0.58%.

Comparative Example A

[0092] Calendered paper (same as layer 1 of Example 1, with a basisweight of 31 g/m²) was prepreged with commercial multi-functional epoxyas in Example 2.

[0093] Thirty-two prepregs made by the above process were furtherlaminated between two Cu sheets (17 μm thick) under the followingconditions in a vacuum press:

[0094] (a) Held in vacuum (no external pressure or temperature).

[0095] (b) Heated to 200° C. (5° C./min) from ambient temperature undera pressure of 6.9 MPa.

[0096] (c) Held for 1 h at 200° C. and 6.9 MPa.

[0097] (d) Cooled to room temperature fast (water quench) under pressure

[0098] Epoxy resin content in the polymer portion of the final laminatewas about 53%. After etching of copper foil, properties of polymerportion of the laminate were measured. CTE in plane was about 12.9 ppm/°C. and moisture absorption at 85° C. and 85% humidity was about 2.1weight %.

Example 3

[0099] LCP blown film (with nominal thickness of 30 μm and average basisweight of 41 g/m²) was laminated between two layers of calendered paper(same as layer 1 of Example 1, with basis weight of 31 g/m²) underfollowing conditions in a vacuum press:

[0100] (a) Held in vacuum (no external pressure or temperature).

[0101] (b) Heated to 320° C. (5° C./min) from ambient temperature undera pressure of 6.9 MPa.

[0102] (c) Held for 1 h at 320° C. and 6.9 MPa.

[0103] (d) Cooled to room temperature fast (water quench) underpressure.

[0104] Laminates were produced with average dimension of 25 cm×20 cm×90μm and average weight of 4.6 g (basis weight 94 g/m²). The laminateswere approximately 60% aramid and 40% LCP.

[0105] The laminates were prepregged with commercial multi-functionalepoxy as in Example 2. The resulting three component composite(aramid+LCP film+epoxy) was 150 μm thick with an average epoxy contentof 25% by weight of the total laminate. The final laminate hadapproximately 45% aramid, 30% LCP film and 25% epoxy, by weight.

[0106] Six prepregs made by the above process were further laminatedbetween two Cu sheets (17 μm thick) under the following conditions in avacuum press:

[0107] (a) Held in vacuum (no external pressure or temperature).

[0108] (b) Heated to 200° C. (5° C./min) from ambient temperature undera pressure of 6.9 MPa.

[0109] (c) Held for 1 h at 200° C. and 6.9 MPa.

[0110] (d) Cooled to room temperature fast (water quench) under pressure

[0111] The laminates had an average thickness of 600 μm (6 prepregs+2 Cusheets)

Example 4

[0112] LCP blown film (with nominal thickness of 20 μm and average basisweight of 28 g/m²) was laminated between two layers of paper (same aslayer 1 of Example 1, with basis weight of 68 g/m²) under followingconditions in a calender:

[0113] Two aluminum foil sheets with shiny side inside (non-stick) wereplaced on the outside of the aramid paper layers of the three layerstack, and the resulting 5 layer stack was calendered at 295° C. and175,000 N/m pressure.

[0114] The laminates had an average basis weight of 220 g/m². Thelaminates were approximately 85% aramid and 15% LCP film by weight.

Example 5

[0115] Teflon®-PFA film (a copolymer of tetrafluoroethylene andperfluoro(propyl vinyl ether) available from E. I. DuPont de Nemours &Co., Wilmington, Del., U.S.A., with nominal thickness of 12 μm andaverage basis weight of 30 g/m².) was laminated between two layers ofpaper (similar to mat in Layer 1 of Example 1, but with a basis weightof 31 g/m²) under following conditions in a vacuum press:

[0116] held in vacuum (no external pressure or temperature)

[0117] heated to 305° C. (5° C./min) from ambient condition under apressure of 6.9 MPa

[0118] held for 1 h

[0119] cooled rapidly to room temperature (water quench) under pressure.

[0120] Laminates were produced with average dimension of 25 cm×20 cm×170μm and average weight of 4.2 g (basis weight 90 g/m²). The laminateswere approximately 65% aramid and 35% Teflon® PFA by weight.

[0121] These laminates from were prepregged with commercialmulti-functional epoxy as described in Example 2. The resulting threecomponent composite (paper+PFA film+epoxy) was 220 μm thick with anaverage epoxy content of 48% by weight of the total laminate. The finallaminate had approximately 34% aramid, 18% PFA and 48% epoxy by weight.

[0122] Two of these prepregs were further laminated between two sheetscopper (17μ thick) under following conditions in a vacuum press:

[0123] held in vacuum (no external pressure or temperature)

[0124] heated to 200° C. (5° C./min) from ambient condition under apressure of 4.1 MPa

[0125] held for 1 hr.

[0126] cooled rapidly to room temperature (water quench) under pressure.

[0127] Laminates were produced with average thickness of 330 μm (2prepregs+2 Cu sheets)

Example 6

[0128] Tefzel® film (a tetrafluoroethylene/ethylene copolymer availablefrom E.I. DuPont de Nemours & Co., Inc, Wilmington, Del., U.S.A., withnominal thickness of 25 μm and average basis weight of 45 g/m²) waslaminated between two layers of aramid paper (same as Layer 1 of Example1, with basis weight of 68 g/m²) under the following conditions in avacuum press:

[0129] held in vacuum (no external pressure or temperature)

[0130] heated to 260° C. (5° C./min) from ambient condition under apressure of 6.9 MPa

[0131] held for 1 h

[0132] cooled rapidly to room temperature (water quench) under pressure.

[0133] Laminates were produced with average dimension of 25 cm×20 cm×350μm and average weight of 9.4 g (basis weight 180 g/m²). The laminateswere approximately 75% aramid and 25% Tefzel®.

[0134] The above laminates were prepregged with commercialmulti-functional epoxy using techniques standard in the industry. Theresulting three component composite (paper+Tefzel®+epoxy) was 520 μmthick. The final laminate had approximately 40% aramid, 13%fluoropolymer and 47% epoxy by weight.

[0135] Two of the above prepregs were further laminated between twosheets copper (17 μm thick) under following conditions in a vacuumpress:

[0136] held in vacuum (no external pressure or temperature)

[0137] heated to 200° C. (5° C./min) from ambient condition under apressure of 4.1 MPa

[0138] held for 1 hr

[0139] cooled rapidly to room temperature (water quench) under pressure.

[0140] Laminates were produced with average thickness of 600 μm (2prepregs+2 Cu sheets)

Example 7

[0141] Two (2.00) g of poly(p-phenylene terephthalamide) fiber wasplaced in a laboratory mixer (British pulp evaluation apparatus) with2500 g of water and agitated for 3 min. Independently, 69.13 g of anaqueous, never-dried, poly(m-phenylene isophthalamide) fibrid slurry(0.43% consistency and freeness 330 ml of Shopper-Riegler) was placed ina same type of laboratory mixer together with about 2000 g of water andagitated for 1 min. Both dispersions were poured together into anapproximately 21×21 cm handsheet mold and mixed with addition of about5000 g of water. The resulting slurry had the following percentages (oftotal solids) of solid materials:

[0142] poly(m-phenylene isophthalamide) fibrids 13%;

[0143] poly(p-phenylene terephthalamide) floc 87%.

[0144] A wet-laid sheet was formed. The sheet was placed between twopieces of blotting paper, hand couched with a rolling pin, and dried ina hand sheet dryer at about 190° C. The dried sheet had basis weight ofabout 53.0 g/m². Another (second) handsheet was prepared exactly asdescribed above. Its basis weight after drying was 52.4 g/m².

[0145] Two (2.00) g of poly(p-phenylene terephthalamide) fiber wasplaced in a laboratory mixer (British pulp evaluation apparatus) with2500 g of water and agitated for 3 min. Independently, 69.13 g of anaqueous, never-dried, poly(m-phenylene isophthalamide) fibrid slurry(0.43% consistency and freeness 330 ml of Shopper-Riegler) was placed ina same type of laboratory mixer together with 2.25 g of 40 meshparticulate LCP and about 2000 g of water and agitated for 1 min. Bothdispersions were poured together into an approximately 21×21 cmhandsheet mold and mixed with addition of about 5000 g of water. Theresulting slurry had the following percentages (of the solids present)of solid materials:

[0146] poly(m-phenylene isophthalamide) fibrids 6.5%;

[0147] poly(p-phenylene terephthalamide) floc 43.5%; and

[0148] particulate LCP 50%.

[0149] A wet-laid sheet was formed. The sheet was placed between twopieces of blotting paper, hand couched with a rolling pin, and dried ina hand sheet dryer at about 190° C. The sheet had basis weight of about98.3 g/m².

[0150] After that all three formed and dried sheets (two withoutparticulate LCP and one with particulate LCP) were calendered togetherbetween two metal rolls of about 20.3 cm diameter at about 350° C. andlinear pressure of about 3000 N/cm, while the sheet with particulate LCPwas used as an inside layer and the sheets without particulate LCP wereused as outside layers. The final calendered sheet had basis weight ofabout 204 g/m², thickness of about 234 μm and a density of about 0.87g/cm³ with maximum concentration of LCP in the middle and practically noLCP on the outside surfaces of the sheet.

[0151] The 40 mesh particulate LCP was prepared by rough grinding an LCPhaving the composition of Example 9 of U.S. Pat. No. 5,110,896 derivedfrom hydroquinone/4,4′-biphenol/terephthalicacid/2,6-naphthalenedicarboxylic acid/4-hydroxybenzoic acid in molarratio 50/50/85/15/320, and which also contained 30% by weight glassfiber, and was in the form of resin pellets (right circular cylindersapproximately ⅛″ in diameter and length) in a hammer mill with liquid N₂also present, and with a coarse (about 10 mesh) discharge screen. Thecourse cut resin was then placed back in the hammer mill with additionalliquid N₂ until the final product passed through a 40 mesh screen.

Comparative Example B

[0152] Two (2.00) g of poly(p-phenylene terephthalamide) fiber (seeExample 7) was placed in a laboratory mixer (British pulp evaluationapparatus) with 2500 g of water and agitated for 3 min. Independently,69.13 g of an aqueous, never-dried, poly(m-phenylene isophthalamide)fibrid (see Example 7) slurry (0.43% consistency and freeness 330 ml ofShopper-Riegler) was placed in a same type of laboratory mixer togetherwith 2.25 g of LCP powder and about 2000 g of water and agitated for 1min. Both dispersions were poured together into an approximately 21×21cm handsheet mold and mixed with addition of about 5000 g of water. Theresulting slurry had the following percentages (as a percent of totalsolids) of solid materials:

[0153] poly(m-phenylene isophthalamide)fibrids 6.5%;

[0154] poly(p-phenylene terephthalamide) floc 43.5%; and

[0155] LCP powder 50%.

[0156] A wet-laid sheet was formed. The sheet was placed between twopieces of blotting paper, hand couched with a rolling pin, and dried ina hand sheet dryer at about 190° C. After that the sheet was calenderedbetween two metal rolls of about 20.3 cm diameter at about 350° C. andlinear pressure of about 3000 N/cm. The final sheet had basis weight ofabout 94.6 g/m², thickness of about 104 μm and density of about 0.91g/cm³ with a uniform distribution of LCP through all the structure.

Example 8

[0157] Poly(p-phenylene terephthalamide) fiber (0.84 g) was placed in alaboratory mixer (British pulp evaluation apparatus) with 2500 g ofwater and agitated for 3 min. Independently, 65.12 g of an aqueous,never-dried, poly(m-phenylene isophthalamide) fibrid slurry (0.43%consistency and freeness 330 ml of Shopper-Riegler) was placed in a sametype of laboratory mixer together with 1.68 g of PET floc and about 2000g of water and agitated for 1 min. Both dispersions were poured togetherinto an approximately 21×21 cm handsheet mold and mixed with anadditional about 5000 g of water. The resulting slurry had the followingpercentages (of total solids) of solid materials:

[0158] poly(m-phenylene isophthalamide) fibrids 10%;

[0159] poly(p-phenylene terephthalamide) floc 30%;

[0160] PET floc 60%

[0161] A wet-laid sheet was formed. The sheet was placed between twopieces of blotting paper, hand couched with a rolling pin, and dried ina hand sheet dryer at about 190° C. The dried sheet had basis weight ofabout 67.0 g/m². The sheet was placed between two sheets of calenderedaramid paper described in example 2. and compressed together in theplaten press under the following cycle: 266° C.-0.21 MPa-2 min.>>266°C.-15.9 MPa-2 min.>>93° C.-15.9 MPa-2 min.

[0162] The final sheet had basis weight of 135 g/m², thickness 0.162 mmand density 0.83 g/cm³ with maximum concentration of poly(ethyleneterephthalate) at the center (of thickness) of the sheet and practicallynone of the PET at the outside surface, as observed using an opticalmicroscope.

Example 9

[0163] Glass fiber (1.26 g) was placed in a laboratory mixer (Britishpulp evaluation apparatus) together with 18.06 g of an aqueous,never-dried, poly(m-phenylene isophthalamide) fibrid slurry (0.43%consistency and freeness 330 ml of Shopper-Riegler), 2.52 g of 30 meshhammer milled LCP pulp prepared as in Example 7 except the final screenwas 30 mesh instead of 40 mesh. The LCP composition was that of Example4 of U.S. Pat. No. 5,110,896, and about 2000 g of water and agitated for1 min. The dispersion was poured into an approximately 21×21 cmhandsheet mold and mixed with an additional about 5000 g of water. Theresulting slurry had the following percentages (of total solids) ofsolid materials:

[0164] poly(m-phenylene isophthalamide) fibrids 10%;

[0165] glass fiber 30%;

[0166] LCP pulp 60%

[0167] A wet-laid sheet was formed. The sheet was placed between twopieces of blotting paper, hand couched with a rolling pin, and dried ina hand sheet dryer at about 190° C. Two other handsheets were preparedby the same procedure, but using 3.50 g of glass fiber and 146.5 g ofthe fibrid slurry for each of them. All three sheets (the sheet with LCPpulp in the middle) were compressed together in the platen press underthe following cycle: 349° C.-0.21 MPa-2 min.>>349° C.-15.9 MPa-2min.>>149° C.-15.9 MPa-2 min.

[0168] The final sheet had a basis weight of 325 g/m², thickness 0.382mm, density 0.85 g/cm³ with maximum concentration of LCP at a center (ofthickness) of the sheet and practically no LCP at the outer surfaces,based on observation with an optical microscope.

Example 10

[0169] Glass fiber (1.26 g) was placed in a laboratory mixer (Britishpulp evaluation apparatus) together with 146.5 g of an aqueous,never-dried, poly(m-phenylene isophthalamide) fibrid slurry (0.43%consistency and freeness 330 ml of Shopper-Riegler), 2.31 g of powderedPPE and about 2000 g of water and agitated for 1 min. The dispersion waspoured into an approximately 21×21 cm handsheet mold and mixed with anadditional about 5000 g of water. The resulting slurry had the followingpercentages (of total solids) of solid materials:

[0170] poly(m-phenylene isophthalamide) fibrids 10%;

[0171] glass fiber 30%;

[0172] PPE 60%

[0173] A wet-laid sheet was formed. The sheet was placed between twopieces of blotting paper, hand couched with a rolling pin, and dried ina hand sheet dryer at about 190° C. Two other handsheets were preparedby the same procedure, but using 3.36 g of glass fiber and 195.3 g ofthe fibrid slurry for each of them. All three sheets (the sheet with PPEresin in the middle) were compressed together in the platen press underthe following cycle: 327° C.-0.21 MPa-1 min.>>327° C.-5.3 MPa-2min.>>149° C.-15.9 MPa-2 min.

[0174] The final sheet had a basis weight of 312 g/m², a thickness of0.428 mm, a density of 0.73 g/cm³, with maximum concentration of PPEresin at the center (of thickness) of the sheet and almost no PPE resinat both outside surfaces.

Example 11

[0175] Pellets of Teflon® polymer of tetrafluoroethylene andperfluoro(propyl vinyl ether) available from E. I. DuPont de Nemours &Co., Wilmington, Del., U.S.A., were refined on 30.5 cm diameterSprout-Waldron Model 12-2 single rotating disc refiner equipped withplates type C-2976-A in one pass at gap between plates of about 25 μm,feeding speed of about 80 g/min. and continuous addition of water inquantity of about 4 kg of water per 1 kg of the pellets. This was usedfor the stock preparation. Two batches of the stock were prepared. Theslurry of the first batch had the following percentages (as a percent oftotal solids) of solid materials:

[0176] poly(m-phenylene isophthalamide)fibrids (freeness 240 ml ofShopper-Riegler) 10%;

[0177] poly(p-phenylene terephthalamide) floc 90%.

[0178] The slurry of the second batch had the following percentages (asa percent of total solids) of solid materials:

[0179] poly(m-phenylene isophthalamide)fibrids (freeness 240 ml ofShopper-Riegler) 8%;

[0180] PFA 92%.

[0181] Two-ply forming was conducted on an inclined wire papermakingmachine with feeding of the slurry of the first batch into the primaryheadbox and feeding of the slurry from the second batch into thesecondary headbox. Headbox consistency in the primary headbox was about0.01% and headbox consistency in the secondary headbox was about 0.1%.Forming was conducted at speed of about 24 m/min, and the basis weightof the formed two-ply paper was about 55.3 g/m², with the first ply(with poly(m-phenylene isophthalamide)fibrids) having a basis weight ofabout 28.1 g/m², while the second ply (with PFA) having a basis weightof about 27.2 g/m² . The formed paper was calendered in two plies withthe PFA-rich plies being in the middle face-to-face. Calendering wasconducted between two metal rolls 20 cm in diameter. The rolltemperature was about 300° C., linear pressure about 1300 N/cm and speedabout 5 m/min. The calendered paper had a basis weight of about 91.2g.m², a thickness of about 0.128 mm, and a density of about 0.72 g/cm³,with a greater concentration of PFA at the center (of thickness) of thesheet and a lower concentration of PFA at outer surfaces, as observedwith an optical microscope. The paper was prepreged with a commercialmulti-functional epoxy described above using standard techniques knownin the industry. Epoxy resin content in the prepreg was about 37 wt. %.The final copper clad laminates containing 1, 2, and 16 plies ofprepreged material were produced in a vacuum platen press underconditions described in Example 2. These laminates had about 28 weight %of PFA, about 35 weight % of aramid components, and about 37 weight % ofepoxy resin based on total weight of polymeric components (but notincluding the copper).

Example 12

[0182] Pellets of LCP were refined in a 91.4 cm diameter Sprout-WaldronModel 36-2 single rotating disc refiner equipped with plates type 16808at feeding speed of about 1.5 kg/min., with addition of about 98.8 kg ofwater per kg of the pellets. After a first pass, pulp produced wasdiluted to consistency of about 0.8 wt.% and refined a second time withdouble recirculation of the slurry through the refiner at gap betweenplates of about 0.25 mm. Refined LCP pulp was screened through anAhlstrom F1 Master Screen with slots 0.36 mm wide. Two batches of thestock were prepared. The slurry of the first batch had the followingpercentages (as a percent of total solids) of solid materials:

[0183] poly(m-phenylene isophthalamide)fibrids (freeness 240 ml ofShopper-Riegler) 15%;

[0184] poly(p-phenylene terephthalamide) floc 85%.

[0185] The slurry of the second batch had the following percentages (asa percent of total solids) of solid materials:

[0186] poly(m-phenylene isophthalamide)fibrids (freeness 240 ml ofShopper-Riegler) 10%;

[0187] LCP pulp 90%.

[0188] Two-ply forming was conducted on an inclined wire papermakingmachine with feeding of the slurry of the first batch into the primaryheadbox and feeding of the slurry from the second batch into thesecondary headbox. Headbox consistency in the primary headbox was about0.01% and headbox consistency in the secondary headbox was about 0.1%.Forming was conducted at a speed of about 30 m/min. Basis weight offormed two-ply paper was about 46.10 g/m², with the first ply (withpoly(m-phenylene isophthalamide)fibrids) being about 29.4 g/m² and thesecond ply (with LCP) being about 16.7 g/m². The paper was calendered intwo plies with LCP-rich plies being in the middle face-to-face.Calendering was conducted between two metal rolls 86 cm diameter each intwo passes. During the first pass the roll temperature was about 340°C., linear pressure about 7300 N/cm and speed about 30 m/min. During thesecond pass, the roll temperature was 60° C., the sheet was heated inthe oven before the nip to a temperature of about 200° C., linearpressure was about 7600 N/cm and the speed was about 15 m/min. Thecalendered paper had a basis weight of about 94.6 g/m², a thickness ofabout 0.104 mm, and a density of about 0.91 g/cm³, with a greaterconcentration of LCP at the center of the sheet and a lowerconcentration of LCP at the outer surfaces, as observed using an opticalmicroscope.

[0189] The paper was corona treated on both sides at a power density ofabout 490 dynes and residence time under the electrode about 0.42seconds. The corona treated paper was prepreged with a commercialmulti-functional epoxy described above using standard techniques knownin the industry. Epoxy resin content in the prepreg was about 40 wt. %.The final copper clad laminates containing 1, 2, and 16 plies ofprepreged material were produced in the vacuum platen press under theconditions described in Example 2. These laminates had about 20 weight %of LCP, about 40 weight % of aramid components, and about 40 weight % ofepoxy resin based on the total weight of polymeric materials in thelaminate (not including the copper).

Example 13

[0190] Strand cut pellets of LCP were refined on a 30.5 cm diameterSprout-Waldron type C-2976-A single rotating disc refiner equipped withplates in one pass at gap between plates of about 25 μm, feeding speedof about 60 g/min. and continuous addition of water in quantity of about4 kg of water per 1 kg of the pellets. This LCP pulp was additionallyrefined with a Bantam® Micropulverizer, Model CF, to pass through a 60mesh screen. A slurry was prepared by mixing the LCP pulp with PPTA. Theresulting slurry had the following percentages (as a percent of totalsolids) of solid materials:

[0191] LCP pulp 90%;

[0192] PPTA floc 10%.

[0193] A continuous sheet (1) was formed from the slurry on a Rotonier(combination of Rotoformer and Fourdrinier) papermaking machine equippedwith horizontal thru-air drier.

[0194] The headbox consistency was about 0.01%, forming speed about 5m/min and temperature of air in the drying section of about 338° C. Thesheet (1) had a basis weight of 68.1 g/m², a thickness of 0.443 mm, andan apparent density of 0.155 g/ml.

[0195] A continuous sheet (2) was formed on the same papermaking machinefrom a slurry containing PPTA floc only. A metal calender roll (diameterabout 15 cm) was placed between the drying section and the wind-upstand, so one-step forming and calendering in-line were conducted. Theheadbox consistency was about 0.01%, line speed was about 5 m/min,temperature of air in the drying section was about 180° C., temperatureof work rolls of the calender was about 350° C. and linear pressure inthe nip was about 1000 N/cm.

[0196] Sheet (2) had a basis weight of 95.6 g/m², a thickness 0.106 mm,and an apparent density of 0.90 g/ml.

[0197] A piece 25×21 cm was cut from the sheet (1), covered on bothsides with an aluminum foil treated with a mold release (Mono-Coat® 327Wsold by Chem-Trend Inc.) and placed in a platen press MTP-20 (sold byTetrahedron Associates, Inc.) between two brass cover plates each 1 mmthick. The sheet was compressed in the press under the followingconditions:

[0198] temperature 350° C., pressure 430 kPa for 1 min;

[0199] cooling to 150° C. at constant pressure of 430 kPa.

[0200] The compressed sheet (1) had a thickness of 0.056 mm and anapparent density of 1.22 g/ml.

[0201] A piece 25×21 cm of compressed sheet (1) was placed between two25×21 cm pieces of sheet (2). The three-ply structure was covered onboth sides with an aluminum foil treated with the mold release andplaced in the platen press between two brass cover plates as describedabove. The lamination of the three plies was conducted under thefollowing conditions:

[0202] temperature 350° C., pressure 170 kPa for 1 min;

[0203] cooling to 150° C. at constant pressure of 170 kPa.

[0204] The final sheet had a basis weight of 259 g/m², a thickness of0.253 mm, and an apparent density of 1.02 g/ml, with a maximumconcentration of LCP at the center (of thickness) of the sheet andalmost no LCP at both outside surfaces, as observed by opticalmicroscopy.

What is claimed is:
 1. A sheet, comprising: (a) one or more nonwovenlayers comprising short lengths of high tensile modulus fibers, and (b)a thermoplastic polymer having low moisture absorption; wherein at leasta portion of said thermoplastic polymer is bound to at least some ofsaid high tensile modulus fibers; and through a cross section of thethickness of said sheet, a concentration of said thermoplastic polymerhaving low moisture absorption, relative to a total concentration ofsaid high tensile modulus fibers is greater at a center of thickness ofsaid sheet than at an outer surface of said sheet.
 2. The sheet asrecited in claim 1 wherein said thermoplastic polymer is a thermotropicliquid crystalline polymer.
 3. The sheet as recited in claim 1 whereinsaid high tensile modulus fibers comprise an aramid.
 4. The sheet asrecited in claim 2 wherein said high tensile modulus fibers comprise anaramid.
 5. The sheet as recited in claim 3 wherein said thermoplasticpolymer comprises a perfluoropolymer.
 6. The sheet as recited in claim 1wherein said high tensile modulus fibers comprise fibers and fibrils. 7.The sheet as recited in claim 1 wherein said high tensile modulus fiberscomprise fibers and fibrids.
 8. The sheet as recited in claim 1 whereinsaid high tensile modulus fibers are fibers only.
 9. A structure,comprising, one or more sheets according to claim 1, and an uncuredthermoset resin which is impregnated into, and coats, said one or moresheets.
 10. The structure as recited in claim 9, further comprising atleast one metal sheet.
 11. The structure as recited in claim 9, whereinsaid uncured thermoset resin is subsequently cured.
 12. The structure asrecited in claim 10, wherein said uncured thermoset resin is cured. 13.A circuit board, chip package, chip carrier, or chip package interposercomprising the sheet of claim
 1. 14. A circuit board, chip package, chipcarrier, or chip package interposer comprising the structure of claim11.
 15. A circuit board, chip package, chip carrier, or chip packageinterposer comprising the structure of claim
 12. 16. A process for theproduction of a sheet material, comprising, heating and applyingpressure to a multilayer structure, comprising at least two outer layersof a nonwoven fabric of short high tensile modulus fibers and at leastone inner layer containing a thermoplastic with low moisture absorption,disposed between said outer layers to produce said sheet material inwhich: at least a major portion of said thermoplastic polymer is boundto at least some of said of high tensile modulus fibers; and through across section of said sheet material, from the center of thickness ofsaid first sheet to both of the surfaces of said sheet material, aconcentration of said thermoplastic polymer relative to a concentrationof high tensile modulus fibers, decreases.
 17. The process as recited inclaim 16, additionally comprising the step of impregnating and coatingsaid sheet material with a thermoset resin and curing said thermosetresin.
 18. The process as recited in claim 17 wherein at least onesurface of said sheet impregnated and coated with said thermoset resinis contacted with a layer of a metal.
 19. The process as recited inclaim 17 wherein said thermoset resin is an epoxy resin.